Composite acoustic decoupler

ABSTRACT

An acoustic decoupler which isolates both shear and longitudinal acoustic waves comprises a body of transversely isotropic low acoustic impedance material with a thin layer of elastomer material attached to one surface of the body of low acoustic impedance material.

United States Patent 1191 Higgs 1 1 June 5, 1973 541 COMPOSITE ACOUSTICDECOUPLER 1,817,086 8/1931 Lindsay et al. ..1s1 33 G 1,880,153 9/1932Rosenzweig [75] Invent 5 9'? Orchard Lake 1,944,533 1/1934 Strobel ..1s133 0 1c 3,215,225 11/1965 Kirschner ..340/5 A [73] Assignee: HoneywellInc., Minneapolis, Minn. [22] Filed: Feb. 10 1972 PrimaryExaminerStephen J. Tomsky Attorney- Lamont B. Koontz and David R,Fairbarn [21] Appl. No.: 225,054

[57] ABSTRACT [52] CL 181/33 :25;; An acoustic decoupler which isolatesboth shear and [51] Int Cl Glok 7 H041) 11/00 E04b 1/99 longitudinalacoustic waves comprises a body of trans- [58] Field gg Lulu/33 A Gversely isotropic low acoustic impedance material with 340/5 8 S, 1 athin layer of elastomer material attached to one surface of the body oflow acoustic im edance material.

[56] References Cited UNITED STATES PATENTS 7 Claims, 4 Drawing Figures1,549,320 8/1925 Lundin ..l8l/33 G v Y 7 A I. 111111,. "I 11111,, 111.111 12 Patented June 5, 1973 3,131,004

COMPOSITE ACOUSTIC DECOUPLER REFERENCE TO RELATED PATENT APPLICATIONSReference should be made to an abandoned patent application SerialNumber 225,053 by P. M. DAmico, entitled Acoustic Decoupler, which wasfiled on an even date herewith and which is assigned to the sameassignee as this application.

BACKGROUND OF THE INVENTION This invention relates to an acousticdecoupler useful in sonar systems. In particular, it relates to acomposite acoustic decoupler capable of decoupling both shear andlongitudinal acoustic waves.

In the design of sonar devices, it is often desirable to isolate thetransducer elements acoustically from each other and from the structureof the device. This isolation is normally accomplished by acousticdecoupling materials, which provide acoustic isolation through impedancemismatch and internal attenuation.

The characteristic impedance, pc, of any material may be represented asthe product of the materials density p and sound velocity 0. For a freeplane wave, this is also equal to the specific acoustic impedance. Whenthe acoustic impedance of one material is the same as that of anadjacent material, the materials are said to be matched; when theacoustic impedances of the two materials are different, the materialsare mismatched.

Two of the desired properties of an acoustic decoupler are first, aninsertion loss of at least 30 db per centimeter through a combination ofreflection and absorption; and second, stable acoustical and mechanicalproperties with temperatures from C to 30C and with pressures oruniaxial stresses, or both, up to 10,000 psi.

In a copending patent application Ser. No. 225,149 by P. M. DAmico andR. W. Higgs, which was filed on an even date herewith, an acousticdecoupler material satisfying these requirements is described. Thismaterial is balsa wood which has been precompressed to a precompressionpressure of between about 2,500 psi and about 20,000 psi. As describedin the copending patent application, precompressed balsa wood is theonly known acoustic decoupler material which has been able to satisfythe above-mentioned requirements, although a large variety of materialshave been used as acoustic decoupler materials.

Despite the many advantages of precompressed balsa wood, it does haveone disadvantage as an acoustic decoupler. Balsa wood has been found tohave transverse isotropy; the sound velocity along balsa woods fiberstructure (or grain) being greater than times the sound velocity indirections normal to the grain. It is believed that this transverseisotropy is the result of the unidirectional grain or fiber structure ofbalsa wood. Experimental results indicate that the sound velocity ofacoustic waves having particle motion normal to the grain structure ismuch less than those acoustic waves having particle motion parallel tothe grain structure. Therefore, sound propagation in the direction ofthe fiber structure must be avoided in decoupling applications. This isachieved by orienting the precompressed balsa wood body such that thelongitudinal acoustic waves impinging on the decoupler are essentiallynormal to the fiber structure of balsa wood. While this effectivelydecouples longitudinal acoustic waves, shear waves can pass through thedecoupler with very little absorption.

SUMMARY OF THE INVENTION The acoustic decoupler of the present inventiondecouples both shear and longitudinal acoustic waves. A transverselyisotropic low acoustic impedance material is oriented such that thedirection of minimum acoustic propagation is normal to the surfaces ofthe body and the direction of maximum acoustic propagation is parallelto the surfaces of the body. A thin layer of elastomeric material isattached to one surface of the low acoustic impedance material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows one embodiment of thecomposite acoustic decoupler of the present invention.

FIGS. 2a and 2b show top and cross sectional views of a sonar systemincluding the composite acoustic decoupler of the present invention.

FIG. 3 shows a multiple section composite decoupler.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 is shown oneembodiment of the acoustic decoupler of the present invention. The bodyof transversely isotropic low acoustic impedance material 10 has a thinlayer of elastomeric material 12 attached to one surface. In thepreferred embodiment of the present invention, the transverselyisotropic low acoustic impedance material is precompressed balsa woodwhich has been precompressed to between about 2,500 psi and about 20,000psi. While precompressed balsa wood is the preferred material due to itslow acoustic impedance and pressure insensitivity, other transverselyisotropic materials such as Sonite, manufactured by Johns ManvilleCompany, can also be used. Sonite is the registered trademark of theJohns Manville Company material designed for underwater soundapplications.

When precompressed balsa wood is used, the fiber structure is orientedessentially parallel to the surfaces of the acoustic decoupler. In thismanner, the direction of minimum acoustic propagation is essentiallynormal to the surfaces of the body, and the direction of maximumpropagation is essentially parallel to the surfaces. To achieve maximumdecoupling of longitudinal waves, the thickness of the precompressedbalsa wood is nh/4, where n is an odd integer and A is the wavelengthcalculated from the velocity of acoustic waves in the balsa wood at themidband frequency of the sonar system.

As described previously, although precompressed balsa wood effectivelydecouples longitudinal acoustic waves, shear waves propagate through thebody with little attenuation. Therefore, the thin elastomeric layer 12is attached to one surface of the balsa wood body. In one preferredembodiment of the present invention, elastomeric layer 12 is a thincoating of silicone rubber such as RTV-l l2 silicone rubber. However,other elastomeric materials such as neoprene rubber, polyurethanerubber, and black gum rubber may also be used. The transmissioncoefficient for shear waves propagating between rigid materials (highshear wave velocity) and elastomeric materials (very low shear wavevelocity) is very low; therefore shear waves, as well as longitudinalwaves, are decoupled by the composite decoupler of the presentinvention.

As described previously, precompressed balsa wood is highly advantageousacoustic decoupler material due to its pressure insensitivity. Theaddition of a thin layer of elastomeric material to form a compositeacoustic decoupler does not adversely affect the pressure insensitivityof the acoustic decoupler, although the elastomeric layer does exhibitsome pressure sensitivity. In particular, the longitudinal soundvelocity-pressure coefficient for elastomers is approximately 0.05m/sec-psi for longitudinal waves. Therefore, a pressure change of 1,000psi increases the sound velocity in the elastomeric material by only 50m/sec. Since the elastomeric layer" is quite thin, the effect of thepressure "sensitivity is very slight.

FIGS. 2a and 2b show top and cross sectional views of a sonar assemblyin which the composite acoustic decoupler of the present invention isused. Transducers 20a and 20b are typically made from piezoelectric orpiezomagnetic materials. Rubber window 22 protects the transducerassembly and improves coupling and directivity of the sound waves. Thematching elements 24a and 24b are. typically a quarter wavelengthsection of aluminum. Since aluminum has a characteristic impedance whichis almost the geometric mean of the impedances of a typicalpiezoelectric crystal and a rubber window, and since the thickness ofeach aluminum matching element is one quarter wavelength, aluminummatching elements 24a and 24b greatly improve the coupling betweencrystals 20a and 20b and the water.

The composite decoupler is positioned between aluminum matching elements24a and 24b and steel support 28. Although elastomer layer 12 is shownadjacent aluminum matching elements 24a and 24b it is to be understoodthat the composite decoupler is equally effective when elastomeric layer12 is adjacent steel support 28. A large acoustic impedance mismatchoccurs between acoustic matching elements 24a and 24b and the compositedecoupler. Therefore, very little of the longitudinal acoustic wavesproduced by the transducers propagate into steel support 28. Asdescribed previously, elastomeric layer 12 attenuates shear waves suchthat both longitudinal and shear waves are decoupled by the compositedecoupler.

If it is desired to increase the insertion loss of the compositedecoupler at low frequencies, multiple sections of the basic compositedecoupler shown in FIG. 1 can be used. FIG. 3 shows a multiple sectionof composite decoupler having alternating layers of precompressed balsawood a and 10b and elastomer material 12a and 12b. 7 r 7 It is readilyapparent to those skilled in the art that many modifications to thepresent invention are possible. It should therefore be understood thatthe invention is not to be limited by the embodiments shown, but only bythe scope of the attached claims.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:

1. In an acoustic system having an acoustic transducer for producingacoustic waves, and having support means for supporting the acoustictransducer, an acoustic decoupler for acoustically isolating theacoustic transducer by decoupling both longitudinal and shear acousticwaves, the acoustic decoupler comprismg:

a body of transversely isotropic low acoustic impedance material havingfirst and second surfaces, the direction of minimum acoustictransmissionbeing essentially normal to the first and second surfaces, and thedirection of maximum acoustic transmission being essentially parallel tothe first and second surfaces, and W V W a thin layer of elastomericmaterial attached to one of the first and second surfaces.

2. The acoustic decoupler of claim 1 wherein the body has a thicknessessentially equal to nA/4, where n is an odd integer and is the acousticwavelength in the low acoustic impedance material calculated at themidband frequency of the longitudinal acoustic waves.

3. The acoustic decoupler of claim 1 wherein the transversely isotropiclow acoustic impedance material is Sonite.

4. The acoustic decoupler of claim 3 wherein the the body has athickness essentially equal to rut/4, where n is an odd integer and )tis the acoustic wavelength in the Sonite claculated at the midbandfrequency of the longitudinal acoustic waves.

5. The acoustic decoupler of claim 1 wherein the transversely isotropiclow acoustic impedance material is balsa wood isostaticallyprecompressed to between about 2,500 pounds per square inch and about20,000 pounds per square inch.

6. The acoustic decoupler of claim 5 wherein the body has a thicknessessentially equal to nit/4, where n is an odd integer and k is theacoustic wavelength in the balsa wood calculated at the midbandfrequency of the longitudinal acoustic waves.

7. The acoustic decoupler of claim 1 wherein the thin layer of elastomermaterial is silicone rubber.

2. The acoustic decoupler of claim 1 wherein the body has a thicknessessentially equal to n lambda /4, where n is an odd integer and lambdais the acoustic wavelength in the low acoustic impedance materialcalculated at the midband frequency of the longitudinal acoustic waves.3. The acoustic decoupler of claim 1 wherein the transversely isotropiclow acoustic impedance material is Sonite.
 4. The acoustic decoupler ofclaim 3 wherein the the body has a thickness essentially equal to nlambda /4, where n is an odd integer and lambda is the acousticwavelength in the Sonite claculated at the midband frequency of thelongitudinal acoustic waves.
 5. The acoustic decoupler of claim 1wherein the transversely isotropic low acoustic impedance material isbalsa wood isostatically precompressed to between about 2,500 pounds persquare inch and about 20,000 pounds per square inch.
 6. The acousticdecoupler of claim 5 wherein the body has a thickness essentially equalto n lambda /4, where n is an odd integer and lambda is the acousticwavelength in the balsa wood calculated at the midband frequency of thelongitudinal acoustic waves.
 7. The acoustic decoupler of claim 1wherein the thin layer of elastomer material is silicone rubber.